SPI/MTD: mtd_dataflash oops prevention

Return a fault code if the Dataflash driver runs into a "no device present"
error when the MISO line has a pulldown (it currently expects a pullup), so
that rmmod won't oops.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>